The evolution of mathematical concepts in tomographic reconstruction and technological advances in instrumentation, and computing in the late 1960s led to the development of x-ray computed tomography (CT). Single-slice, first generation CT scanners became available commercially in the early 1970s for clinical applications for diagnostic imaging of the head. These first generation CT scanners consisted of a pencil beam geometry and a single detector utilizing a translate and rotate acquisition geometry which required several minutes to produce a single cross-sectional image slice (tomogram) through the skull. The publication entitled “Computerized transverse axial scanning (tomography)” authored by Hounsfield, British Journal of Radiology, 1973 December; 46 (552): 1016-22 discloses such a first generation CT scanner.
The impact of CT scanning technology was significant and led to the rapid development of new systems which employed larger x-ray beams and more detectors in order to decrease scan time and increase the area of scan coverage. By the early 1980s, clinical CT scanners could acquire transverse (trans-axial), single cross-sectional images through any part of the human anatomy in a matter of seconds. By the early 1990s, clinical CT scanners combined bed patient motion in the axial direction with gantry rotation speeds in the order of one (1) second per revolution to generate so-called helical data sets. These CT scanners routinely generated stacks of two dimensional (2-D) image slices representing the three dimensional (3-D) volume of the anatomy being scanned. The discrete elements forming the 3-D volume are commonly referred to as voxels. The availability of advanced computing and visualization equipment allowed physicians and scientists to arbitrarily view image slice data through the scanned 3-D volume of tissue.
By the late 1990s, CT scanners were developed with multiple rows of detectors and larger x-ray beam coverage (extending several slices axially), which further decreased the scanning time for imaging large volumes of anatomy. In 2004, CT scanners generating sixty-four (64) slices per gantry rotation became commercially available. In 2005, CT scanners became available that acquire 3-D data sets with the so-called cone-beam geometry (divergent x-ray beam fully extending across the trans-axial direction and covering many slices axially) in a matter of seconds. The 3-D voxel data set generated by these CT scanners is typically reformatted and viewed using a number of display techniques, i.e. multi-planar reformatting, maximum/minimum intensity projection, and volume rendering.
In addition to CT scanning, other volume imaging modalities such as for example, magnetic resonance, ultrasound, and nuclear/PET imaging, have been developed that generate 3-D data sets.
Quantifying the spatial resolution, resolving power, or detail detectability of an imaging system has always been of significant interest and utility, particularly in medical imaging systems. Standard methods of assessing resolution include line-pair or spoked phantoms (test objects) which contain high contrast structures with varying spatial frequencies. These phantoms are 2-D in nature and typically, imaging system performance is evaluated through the visual inspection of phantom image data where the highest spatial frequency for which structures can be discriminated (resolved) indicates the limiting spatial resolution. Line-pair phantoms assess resolution in one dimension only and may be rotated and re-imaged to assess resolution in the axial and trans-axial directions.
In 1976, a method by which a continuous set of spatial frequencies could be examined simultaneously, to examine CT scanner spatial resolution performance was proposed and is described in the publication entitled “The line spread function and modulation transfer function of a computed tomographic scanner” authored by Judy, Medical Physics, 1976, July-August; 3(4): 233-6. The disclosed method involves imaging a high contrast slanted edge trans-axially. The image slice data provides the edge response function (ERF), which in turn provides the response of the CT scanner to a step function. Differentiating the ERF generates a point spread function (PSF) and thus, the response of the CT scanner to an impulse (delta) function. Since the impulse function contains all frequencies, this means that the response of the CT scanner to all spatial frequencies may be assessed, in one dimension, simultaneously, by imaging the slanted edge.
The ‘slanting’ of the edge in a 2-D image allows for over-sampling, thereby overcoming the limitations of discrete sampling inherent to digital data sets. The edge may be rotated in the scanning field of the CT scanner to measure the resolution in the ‘left-right’ and ‘anterior-posterior’ directions of trans-axial images. The edge may be rotated again to measure axial resolution.
Assessment of resolution in a CT scanner for quality control purposes has typically been done using a bar pattern phantom with a series of stacked plates aligned with the axis of the CT scanner. While quantitative modulated transfer function (MTF) measures of bar pattern data have been reported, most quantitative measures have been made with wires, thin plates, or the surface of a block. In all cases, the phantoms are aligned with the axis of the CT scanner and the data from a transverse slice is analyzed using traditional methods for assessing the MTF from a point spread function (PSF), line spread function (LSF), or edge spread function (ESF). For these methods, the two-dimensional image data is analyzed using traditional radiographic methods that relate the PSF, LSF, and ESF to the MTF.
The importance of resolution measurements is driven by the interdependent relationship between image quality (noise), resolution, and dose. A novel method for quantifying spatial resolution in a volume micro-CT scanner is described in the publication entitled “Resolving Power of 3D Microtomography Systems” authored by Seifert and Flynn, Medical Imaging 2002, Proceedings of SPIE Vol. 4682 (2002):407-413). The method involves imaging a sphere of uniform material composition immersed in a homogeneous media. The sphere represents a ‘3-D edge’. The sphere and surrounding media are imaged, and the voxel intensity for each voxel, in the resulting volume data set, is plotted as a function of distance from the center of the sphere. This plot represents the ERF in all directions and is referred to as the surface spread function (SSF). The surface spread function is differentiated to produce the equivalent of the PSF, which is subsequently Fourier transformed to produce the modulated transfer function (MTF) for the micro-CT scanner.
For modem multislice CT (MSCT) scanners, which routinely produce three dimensional volumetric data sets with nearly isotropic resolution, methods that can assess the MTF as a function of direction in three dimensions (3D) are needed.
It is therefore an object of the present invention to provide a novel method of evaluating the resolution of a volumetric imaging system and image phantom used during resolution evaluation.